NOTE: The seven-day week originated in ancient Mesopotamia and became part of the Roman calendar in C.E. 321. The names of the days are based on the seven celestial bodies (the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn), believed at that time to revolve around Earth and influence its events. Most of Western Europe adopted the Roman nomenclature. The Germanic languages substituted Germanic equivalents for the names of four of the Roman gods: Tiw, the god of war, replaced Mars; Woden, the god of wisdom, replaced Mercury; Thor, the god of thunder, replaced Jupiter; and Frigg, the goddess of love, replaced Venus.